<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<title>盒子布局实验</title>
<style>
  .box{
    display: flex;
    justify-content: center;
    flex-wrap: wrap;

  }
  #box1,#box2,#box3{
    width: 400px;
    height: 200px;
    border: 5px solid black;
    margin-right: 10px;
    justify-content: center;
  }
  #box4{
    width: 400px;
    height: 200px;
    border: 5px solid black;
    align-self: center;
    margin-top: 10px;
    margin-right: 10px;
  }
</style>
</head>
<body>
    <div class="box">
        <div id="box1"></div>
        <div id="box2"></div>
        <div id="box3"></div>
        <div id="box4"></div>
    </div>
</body>
</html>